Concomitant COVID and Influenza Vaccination Raise the Risk of Stroke

The study, currently a preprint, was conducted by the CDC and FDA using the Vaccine Safety Datalink (VDS). The VDS differs from VAERS in that it gathers information from specific health systems, in this case Medicare beneficiaries, about specific concerns; VAERS, the Vaccine Adverse Event Reporting System, is voluntary, passive surveillance.

The study design is termed self-controlled case series, SCCS. Each participant acts as their own control, thereby eliminating time-independent factors, such as co-morbidities, and each participant has experienced an outcome. The SCCS tries to identify a temporal linkage between, in this instance, the receipt of a COVID or high-dose influenza vaccine and the onset of a stroke. Strokes were further subdivided into non-hemorrhagic, transient ischemic event (TIA), a stroke that resolves without any physical sequelae within 24 hours, and hemorrhagic stroke. The risk intervals for experiencing this adverse outcome were 1-21 days and 22-42 days, with a control interval of 43-90 days after vaccination.

The dataset of Medicare beneficiaries age 65 or older included about 5.4 million individuals vaccinated between August 31, 2022, and November 6, 2022.

The overall stroke incidence for these beneficiaries was 0.2%, which led the researchers to conclude:

“There were no statistically significant associations of an increased risk of stroke following administration of the COVID-19 bivalent vaccines … [and the] temporal scan did not identify any significant clusters for stroke outcomes after either COVID-19 bivalent vaccine.”

• In a sub-group analysis, there was a higher incidence of stroke within the first 21 days for those aged 85 or older receiving the Pfizer vaccine and a higher incidence of stroke between 22 and 42 days for those aged 65 to 74 receiving the Moderna vaccine.
• Those individuals receiving both COVID and influenza vaccine had an even higher rate of stroke.  There was a 20% increase in the risk of non-hemorrhagic stroke for those receiving the Pfizer and influenza vaccine and a 35% increase in the risk of transient ischemic stroke for those who got the Moderna and influenza vaccine.

In light of these findings, the researchers examined the 6.9 million Medicare beneficiaries receiving the high-dose influenza vaccine alone. In this instance, the overall incidence of those four types of stroke was 0.30%. After further sub-group analysis, the researchers concluded:

“We additionally found a slightly elevated risk of stroke following influenza vaccines administered without concomitant COVID-19 bivalent vaccines. This finding suggests that the observed risk of stroke in the concomitant subgroup was likely driven by influenza vaccination alone rather than concomitant administration.” [emphasis added]

Despite the instant reporting by those compelled to comment on studies that may not actually have read on the former Twitter, the anti-vax forces do not need either a victory lap or “I warned you.” And perhaps those who say our vaccine safety surveillance systems are being ignored and that they contain the smoking gun against the COVID vaccines may also want to be a little less strident.

“However, our study results, in combination with the known benefits of COVID-19 and influenza vaccination, do not change the conclusion that the benefits of these vaccines outweigh their risks in persons ≥65 years.”

That conclusion by the researchers is perhaps a bit misleading; after all, there was a slight risk from concomitant vaccination. There was no evidence for an elevated risk when the vaccines were given further apart – the take-home might be that for those at risk, it would be wiser to space those vaccinations out, not avoid them.

Source: Evaluation of Stroke Risk Following COVID-19 mRNA Bivalent Vaccines Among U.S. Adults Aged ≥65 Years MedR_xIV DOI: 10.1101/2023.10.10.23296624